1. Field of the Invention
Embodiments of the present invention relate to a water-based ink set for ink-jet recording containing a black ink (hereinafter, referred to also as “Bk ink”), a yellow ink (hereinafter, referred to also as “Y ink”), a magenta ink (hereinafter, referred to also as “M ink”) and a cyan ink (hereinafter, referred to also as “C ink”).
2. Description of the Related Art
In an ink-jet recording system, ink droplets are formed by using any one of several ink discharging methods including, for example, an electrostatic attraction method, a method in which mechanical vibration or displacement is applied to an ink by using a piezoelectric element, and a method in which bubbles are generated by heating the ink. Then, a part or all of the thus-formed ink droplets are allowed to adhere to a recording material such as paper to effect the recording.
Such ink-jet recording methods typically use an ink set, containing a Y ink, an M ink, a C ink and a Bk ink. Each of these inks typically includes a pigment or a dye dissolved or dispersed in a liquid medium containing water and a water-soluble organic solvent.
Individual inks constituting the ink set typically have the following properties for optimal ink-jet recording:
1) recovery performance upon ink introduction into an ink-jet head after previous ejection failure from a nozzle is excellent;
2) when an image is formed on recording paper, particularly using the Bk ink, high printing quality is obtained; this high quality image has a sharp outline or contour of the image without causing any irregular flow of the ink along a fiber of the recording paper or irregular penetration (hereinafter, referred to also as “blurring”) of the ink into a space between fibers of the recording paper;3) when an image is formed on recording paper, particularly using the Bk ink, ink drying on a surface of paper is excellent;4) when an image is formed on recording paper, although inks having different colors become blended with each other at their boundaries with one another, (hereinafter, such blending phenomenon being referred to as “bleeding”), the bleeding occurs only to a small extent; and5) when an image is formed on recording paper, color unevenness does not occur, or occurs only to small extent in a color printing region.
In order to achieve these properties, a penetrant, such as a surfactant and the like, that enhances a penetrability is often added to the ink. The drying of each ink on the surface of paper is also typically improved. Blurring, however, particularly of the Bk ink, is conspicuous in such inks. On the other hand, when the penetrability of each ink is reduced, there is a tendency for the bleeding, including bleeding between the Bk ink and other ink colors and also between the Y ink and the M ink or between the Y ink and the C ink, is aggravated. In order to improve this bleeding problem, the amount of added surfactant or penetrant may be adjusted, but then not are there problems with Bk ink blurring and drying on the surface of paper, but also the color unevenness in the color printing region tends to be aggravated.
Accordingly, it is difficult to obtain satisfactory recovery performance upon ink introduction into the ink-jet head only by adjusting the penetrability of the ink into the recording material. As described above, there is no ink set for ink-jet recording in related art which simultaneously satisfactorily solves the problems of the recovery performance upon ink introduction into the ink-jet head, the blurring of the Bk ink, Bk ink drying on the surface of paper, the bleeding between the different inks, and color unevenness in the color printing region.